This invention relates to a remote split scan detector, and more particularly, to a remote split scan detector which utilizes two rectangular cross section light pipes to transfer a light beam onto two light detecting sensors. The remote split scan detector of this invention is used in a raster scanning system in order to detect the start of the scan and end of scan positions. Hereinafter "light detecting sensor" will be referred to as "sensor".
Typically a raster output scanner (ROS) needs a start of scan pulse in order to trigger the electronics to send the video stream at a precise time which corresponds to a precise location for the light exposure on a desired medium.
There are different kinds of scan detectors such as single sensor knife edge and dual sensor split scan detector which can be used to detect start of scan and end of scan. Dual sensor split scan detectors are more accurate since they are insensitive to beam intensity variations. Hereinafter, dual sensor split scan detectors will be referred to as "split scan detector". Split scan detectors are usually placed in the ROS housing. Typically, two mirrors are placed in the path of the scanning light beam at the output window of the raster output scanner one prior to the start of scan and the other at the end of scan line. Each mirror reflects the scanning light beam onto one split scan detector. The detector which receives the light beam from the mirror prior to the start of scan detects the start of scan and the detector which receives the light beam from the mirror at the end of scan detects the end of scan. Some systems use only the start of scan detector and some systems use only the end of scan detector.
For the purpose of simplicity, hereinafter a generic scan detector will be discussed since the discussion of the generic scan detector applies to both the start of scan detector and the end of scan detector.
Placing the split scan detector in the housing of the raster output scanner creates some difficulties. The split scan detector has two sensors spaced apart from each other in such a manner that the two adjacent edges are parallel to each other. Typically, the space between the two sensors is less than the diameter of the light beam. Since the sensors are in the path of the scanning light beam, the light beam crosses both sensors. Every time the light beam exits one sensor, crosses the space between the two sensors and enters the second sensor, a pulse will be generated. Each split scan detector has an associating comparator circuit.
The main electronic board is usually outside of the raster output scanner housing. However, the comparator circuit associated with each split scan detector has to be placed in the raster output scanner housing next to each split scan detector. Therefore, two small circuit boards each containing two sensors of a split scan detector and its associated comparator circuit are placed inside the raster output scanner housing. The two detector circuit boards have to be hard wired to the main electronic board.
It is not desirable to have electronic circuits away from the main electronic board since having additional circuit boards adds considerably to the cost of manufacturing.
There has been an attempt to solve this problem by placing detectors on the main electronic board and transferring the reflected scanning light beam onto the electronic board via light pipes. However, in this approach single sensor detectors are used. Since each detector has one sensor, this approach uses one circular cross section light pipe for each detector to transfer the light beam from the ROS housing onto the sensor in the main electronic board and the light pipe is used in such a manner that one end of the light pipe is attached to the sensor and the other end of the light pipe is placed in the ROS housing. Therefore, every time the light beam crosses the circular end of the light pipe in the ROS housing, the light beam will be transferred to the sensor on the main electronic board and the sensor will generate a pulse.
This approach is less accurate than a split scan detector approach because it is sensitive to variations in beam intensity.
It is an object of this invention to transfer the reflected scanning light beam from the ROS housing onto a split scan detector at a different location than the ROS housing with a high accuracy.